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1. Field of Invention
This invention relates to a method for washcoating a catalytic material onto a monolithic structure. More particularly, it relates to a monolithic catalyst produced by washcoating a monolith structure with a catalytic washcoat composition, wherein the catalytic washcoat composition is formed by blending a zeolitic material with a binder system, wherein the binder system is a dual sol binder, preferably a silica sol and alumina sol binder system.
2. Description of Related Art
Monolithic catalyst products having a cellular or honeycomb structure have increasingly been utilized in stationary emission control situations and for chemical synthesis and processing facilities. Monolithic catalyst products are also utilized in the automotive industry and for NOx conversion in feed streams.
Catalyst products of this type are generally fabricated by preparing a substantially homogeneous cellular ceramic material and securing a catalyst material to that ceramic material. The catalyst product can also be formed by impregnating a monolith structure with a catalyst material and then heat treating the coated monolith to produce the final product.
Conventional washcoating techniques for production of catalytic monolith products generally comprise preparing a coating formed from a high surface area oxide blended with one or more catalysts and dipping the monolith structure into that coating blend. In one utilization for automotive converters the catalyst comprises one or more noble metals, such as platinum, palladium and rhodium. These noble metals are blended with high surface area metal oxides, such as alumina or ceria. These washcoats are then coated upon monolithic support structures, such as ceramic honeycomb supports. A method of coating a preformed high surface area washcoat onto a monolithic support is disclosed in U.S. Pat. No. 4,900,712.
Numerous references disclose the use of catalytically active washcoat compositions comprising an alumina binder and catalytically active noble metals for oxidation of hydrocarbons and/or reduction of nitrogen oxides for field abatement, automotive exhaust control and the like, including, for example, U.S. Pat. Nos. 6,150,291, 6,093,378, 6,080,377, 5,773,423 and 5,354,720.
In one specific example of the formation of a catalytic monolithic product for the selective catalytic reduction of NOx, a washcoat is coated onto a honeycomb substrate, such as a cordierite honeycomb substrate. In U.S. Pat. No. 5,206,202 a honeycomb substrate is coated with a catalytic iron/vanadium mixture blended in an alumina or titania binder.
In another example, U.S. Pat. No. 5,776,423 discloses a process for the preparation of a metallic zeolite catalyst for NOx abatement. Example 1 of the ""423 patent discloses a process for the formation of a copper and iron ZSM-5 zeolite product. The binder (Nalco(trademark) 1056) for the metallic zeolite catalyst product is comprised of a 4 weight percent alumina coating on a 26 weight percent silica sol. This example discloses the use of only one type of sol for the washcoat, i.e., a silica sol. The alumina utilized in the example is chemically bonded to the silica sol by means of Alxe2x80x94Oxe2x80x94Si bonds. See also U.S. Pat. Nos. 6,150,291 and 5,354,720.
U.S. Pat. No. 6,093,378 discloses a catalyst composition for treating diesel exhausts which comprises blending a first zeolite component in the presence of a precious metal, a second zeolite component and a precious metal and a non-catalytic, pore-containing zeolite. The binder for the active materials as disclosed in Examples 2 and 4 is apparently the same Nalco(trademark) material that was disclosed in U.S. Pat. No. 5,776,423. See also U.S. Pat. No. 5,443,803.
Other catalysts for purifying exhaust gases are disclosed by U.S. Pat. Nos. 5,443,803 and 5,354,720. Each of these patents discloses the use of a silica sol used as a binder for coating an active metal onto a monolithic structure.
U.S. Pat. No. 6,004,896 discloses a hydrocarbon absorber and a method of making same. A ZSM-5 zeolite is treated with phosphoric acid and then blended into a slurry for washcoating onto a monolithic structure. The preferred binder for the slurry is alumina, although the composition of the binder may also include silica and/or zirconia or their precursors.
JP 1135541 discloses an exhaust gas cleaning catalyst comprising a zeolite which has been ion exchanged with one or more metals selected from the group consisting of Pt, Pd, Rh, Ir and Ru. The ion exchanged zeolite is blended with a binder. The particularly preferred binder, as shown in Working Examples 1-5, is a mixture of a silica sol and an alumina sol obtained by mixing the two components at a Si/Al elemental ratio of 30. With a silicon to aluminum elemental ratio of 30:1, the silica:alumina ratio of the binder is approximately 9:1. The blended product is adjusted to a pH of about 3 to 6 by the addition of an aluminum nitrate solution. Binders with such high silica:alumina ratios have a reduced ability to adhere to monolithic structures and exhibit high cell clogging during application. In addition, while the aluminum nitrate may have been added to the blend for pH adjustment, it is believed that it was also added to enhance the ability of the zeolite to bind to the substrate. The addition of such pH adjusting materials may adversely affect the ability of the blended product to bind to the substrate. The teaching of JP 1135541 is that zeolite-based precious metal catalysts have a superior performance to catalysts formed where alumina is the support for the precious metal. Any advantage from the use of a particular binder blend was not discussed and apparently not recognized.
U.S. Pat. No. 5,813,764 discloses a catalytic microcalorimeter sensor for monitoring exhaust gas conversions. The sensor disclosed is a washcoat obtained by mixing silica sol and alumina sol. The washcoat is used as the catalytic material itself and is not used as a binder material. The ""764 patent also discloses the use of catalytically active metal particles. See the related patent, U.S. Pat. No. 5,707,148.
Conventional zeolitic materials washcoated onto a monolithic substrate, such as a honeycomb monolith, conventionally utilize silica sol or alumina sol separately as a binder. When either silica sol or alumina sol is used separately as a binder, the resulting catalytic washcoat material generally has low viscosity, in the range of 1-50 cps and normally in the range of 10-20 cps. Silica sol is the preferred binder for washcoating zeolite materials onto a monolithic substrate as silica sol is miscible with zeolite. In contrast, when alumina sol is used as a binder, it tends to separate from the zeolite materials. In addition, when silica sol is used as a binder, zeolite/binder mixtures dehydrate very quickly when exposed to air, which can lead to cell clog during washcoating as the washcoat slurry dries before it is removed from the cell during processing. Further, when a silica sol binder is utilized, the pick up of the catalytic washcoat material onto the monolithic structure decreases during consecutive dipping steps. In addition, the catalytic washcoat material often has weak adhesion to the monolithic substrate when a silica sol is used as the binder. Thus, when a single sol binder is utilized, the monolithic structure may lose some of the catalytic washcoat material during utilization from flaking or when it is exposed to a liquid. When alumina sol is used a binder, the resulting washcoat slurry is not stable and the binder tends to separate from the catalytic material.
Accordingly, it is an object of this invention to disclose a process for the coating of a monolithic structure with a catalytic washcoat composition which produces a monolithic catalyst which is stable, reduces clogging, has an increased pick up in consecutive dipping steps, and has an enhanced adhesion of the catalytic washcoat composition to the monolithic structure.
It is a further object of the invention to disclose a catalytic washcoat composition for coating a monolithic structure wherein the binder for the catalytic material is formed from a dual sol mixture.
It is a still further object of the invention to disclose a catalytic washcoat material for coating a monolithic structure, wherein the binder is a dual sol binder and wherein the dual sol binder is a combination of silica sol and alumina sol.
It is a still further object of the invention to disclose a catalytic washcoat material for coating a monolithic structure wherein the binder is a combination of a silica sol and an alumina sol, wherein the ratio of the silica to the alumina in the binder system is 6:1 or less.
It is a still further object of the invention to disclose a catalytic washcoat material for coating a monolithic structure with a zeolitic material wherein the binder system is a combination of a silica sol and an alumina sol which does not require the addition of additive materials to enhance the adhesion of the catalytic washcoat material to the monolithic structure.
It is a still further object of the invention to disclose a monolithic catalyst for selective catalytic reduction utilizing a monolithic structure coated with a catalytic washcoat material comprising a metal exchange zeolite and a dual sol binder system, wherein the dual sol binder system is a combination of a silica sol and an alumina sol.
These and other aspects of the invention are obtained from various designs of the catalyst material and the binder system of the present invention, the process of their manufacture and use and by the processes disclosed herein.
The present invention is a catalytic washcoat composition for coating a monolithic structure comprising a binder system and a catalytic material, wherein the binder system is a dual sol binder system. Preferably, the dual sol binder system is a blend of a silica sol and an alumina sol, wherein the ratio of the silica to the alumina in the binder system is from about 1:3 to 6:1, preferably from about 3:1 to about 6:1.
The present invention also relates to a process for production of a washcoated monolithic catalyst comprising
preparing a monolithic structure,
preparing a catalytic washcoat composition by mixing a binder system with a catalytic material, wherein the binder system comprises a dual sol blend, wherein the dual sol blend comprises a mixture of a silica sol and an alumina sol, and wherein the ratio of the silica to the alumina in the dual sol blend is from 1:3 to 6:1, and
coating the monolith structure with the catalytic washcoat composition.
In a preferred embodiment, the process is performed without the addition of additive materials to enhance the adhesion of the catalytic washcoat composition to the monolithic structure.
The invention also encompasses a process for the reduction of NOx in a feed stream comprising passing the feed stream over a monolithic catalyst, wherein the monolithic catalyst comprises a monolithic structure coated with a catalytic washcoat composition wherein the catalytic washcoat composition comprises a binder system and a catalytic material, wherein the binder system comprises a dual sol binder, wherein the dual sol binder comprises a mixture of a silica sol and an alumina sol, and wherein the ratio of the silica to the alumina in the mixture is from about 1:3 to 6:1, preferably from about 3:1 to about 6:1.